Fix NPE in pausePlayback

Null check mActivity as we do in onPause

Bug:22249553
Change-Id: I147fbd517696205a2eb478a6df032f50cbbd5b34
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 19f7145..22a3a45 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -619,7 +619,9 @@
         }
         mAudioManager.abandonAudioFocus(this);
 
-        mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+        if (mActivity != null) {
+            mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+        }
         disableProximitySensor(true /* waitForFarState */);
     }